This invention relates to fuel cells and, in particular, to methods and apparatus for maintaining a compressive load on stacks of such fuel cells. More specifically, the invention relates to methods and apparatus for maintaining a compressive load in high temperature systems such as molten carbonate and solid oxide fuel cell stacks.
A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery includes a negative (anode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are produced adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate in between each cell.
In the present state of the art, a fuel cell stack may have several hundred cells in series. In order to work properly, intimate contact must be maintained between all cells in the stack. Also this contact must continue during all stack operating conditions for the duration of the stack""s life. Factors to be considered in achieving this requirement include manufacturing tolerances of the cell components, non-uniform thermal expansion of the cell components during operation and long term consolidation of the cell components resulting in shrinkage of the stack.
Accordingly, a variety of requirements are placed on the system used to compress the fuel cell stack. The system must apply enough load to overcome the manufacturing tolerances early in life to bring the cell components into intimate contact. It also must be great enough during operation to prevent the cells from delaminating due to the inevitable thermal gradients within the stack. At the same time, the compression system load should not be so great as to cause excessive shrinkage of the stack during its life as this places undue demands on auxiliary stack hardware and on the required follow-up of the compression system itself An additional requirement is that the system does not completely relax over time to insure that adequate stack pressure is maintained through the end of life.
Conventional fuel cell stack designs use one of a number of mechanisms for applying compressive load to the stack. U.S. Pat. No. 4,430,390 describes spring members which run within the manifolds of the fuel cell stack attaching to the endplates and forcing them toward each other. This design is not desirable for high temperature systems such as molten carbonate and solid oxide stacks because the spring members would need to be excessively large and be constructed of exotic, corrosion resistant materials to withstand the high temperature environment. U.S. Pat. No. 4,692,391 describes a design where the end plates are directly connected by rigid tensile members such as bolts or threaded rods. However, this system provides practically no load following capability to maintain stack compression as it shrinks.
U.S. Pat. No. 5,686,200 describes small, twisted wire or ribbon springs which may be used to apply load to individual cells within a stack. This design is inappropriate for large area fuel cells as the separator plates to which the springs are attached could not be constructed stiff enough to insure adequate load was delivered to the central area of the cells. U.S. Pat. No. 5,789,091 describes the use of continuous compression bands which are wrapped around the stack and placed in tension. Again, this method suffers from inadequate follow-up for stacks with significant long term creep.
Other methods of stack compression commonly used in the field include placing coil or belleville disk springs in compression at the end of a set of stiff tie rods which tie the opposing stack end plates together. Another design utilizes flexible bars to span the opposing end plates which are again connected by rigid rods to form leaf springs. These designs suffer from either inadequate follow-up or inadequate load capability for present state of the art, large scale, high temperature fuel cell stacks.
A more complex system, previously employed by the assignee of the subject application, uses rigid tie bars to span the top end plate. Rigid tie rods are connected to the tie bars and to a mechanical linkage near the bottom of the stack. This linkage connects the tie rods to a spring assembly in the form of a belleville disk pack located under the bottom end plate. Insulating layers are used to protect the spring assembly from excessive temperature during service. In this system the spring and linkage are designed so as to apply a fairly constant load to the stack during its life. Additionally, to accommodate the geometry of the mechanical linkage, the spring assembly is designed to rotate about one of its ends during operation.
This design has the limitations of causing excessive stack shrinkage and requiring extra space under the stack to accommodate rotation of the spring assembly. Another limitation of this design lies in the high cost associated with the numerous, large belleville disks which make up the spring assembly. Finally, the design presents safety concerns, since the disks of the spring assembly must be preloaded prior to installation in order to generate a relatively constant load profile.
It is therefore an object of the present invention to provide a fuel cell stack compression system which overcomes the above disadvantages of the prior art systems.
It is a further object of the present invention to provide a fuel cell stack compression system with desirable load-deflection characteristics applicable to large, high temperature fuel cell stacks;
It is also an object of the present invention to provide fuel cell stack compression system with minimized space requirements under the fuel cell stack;
It is yet a further object of the present invention to provide a fuel cell stack compression system having reduced costs; and
It is also an object of the present invention to provide fuel cell stack compression system which is safe to use.
In accordance with the principles of the present invention, the above and other objectives are realized in a compression system for providing a compressive force to a fuel cell stack in which the compression system includes one or more members each connected to a first end of the stack and extending to the opposite second end of the stack and a coupling mechanism situated adjacent the second end of the stack and including for each of the one or more members a spring assembly and a linkage assembly together adapted to cause translation movement provided by the spring assembly to be converted into a movement of the associated member in a direction between the first and second ends of the stack without rotation of the spring assembly. In this way, a desired compressive force can be maintained between the ends of the stack by the one or more members as the stack geometry changes.
In the embodiment of the invention to be disclosed hereinafter, the spring assembly is horizontally situated under the second end of the fuel cell stack and includes a fixed base plate and a translatable captivating plate between which the one or more springs of the assembly are situated. A shaft extends horizontally through the center of the springs and a first end of the shaft is attached to the captivating plate and a second end engages the linkage assembly. The latter assembly includes a lever arm, first, second and third pins and a slotted bearing . The first pin is rotationally mounted at one end of the lever arm and is connected to the end of the associated member. The second pin serves as the pivot point for the lever arm and is rotationally mounted to a base frame. The third pin is rotationally mounted to the second end of the lever arm and is situated in the slot of the slotted bearing. The bearing, in turn, is engaged by the second end of the shaft of the spring assembly.
With this configuration, the horizontal translation of the one or more springs of the spring assembly is carried to the captivating plate and from the captivating plate to the shaft of the spring assembly. The horizontal translation of the shaft is then carried by the shaft to the bearing and from the bearing to the third pin. This causes the pin to undergo both horizontal translation and translation in the orthogonal direction, i.e. vertical translation, due to rotation of the lever arm about the second pin. These translations are then imparted to the first pin at the first end of the lever. This, in turn, causes the associated member to undergo a vertical translation, which translation maintains the desired compressive force on the fuel cell stack
In a further aspect of the invention, a plurality of concentric springs of different length are used in the spring assembly to realize a desired non-linear, decreasing compressive load on the fuel cell stack.